1. Field of the Invention
This invention relates to variable-speed alternating current (AC) motor drives and a method for operating a variable-speed AC motor drive, and particularly to controlling such a drive during a collapse of the input source voltage.
2. Description of the Prior Art
Many existing variable speed AC motor drives operate from an AC source (such as a utility) that is shared by other equipment. Such a source may experience a brief collapse of voltage occasionally, due to operation of its other loads, or to switching operations by the utility. It is very desirable that the variable speed AC motor drive should continue operating during such a disturbance, but it is economically impractical to store enough energy within the drive to supply the load power throughout the disturbance. Furthermore, the control circuits within the drive may depend on the same stored energy to sustain their operation during the disturbance. If that energy is depleted beyond a certain point, the drive may shut itself down or trip. Many processes can tolerate a temporary loss of speed during a disturbance far better than they can tolerate a drive shutdown. Therefore, a need exists for a drive control method which, upon detecting a collapse of input source voltage, can quickly stop the flow of power, allowing the speed to decrease, but preserving the stored energy for the control circuits. The object of such a control is that the drive should not shutdown during a brief collapse of supply voltage, so that normal operation can quickly resume at the conclusion of the disturbance.
Most drives use capacitors in the DC link to store energy, and to provide smoothing. In the event that the input source voltage is greatly diminished or lost, the capacitors would rapidly become discharged if continued operation of the motor under load is attempted. Thus, it is desirable that the loss of energy from the capacitors be quickly curtailed as soon as a collapse of input source voltage is detected.
Several schemes are known in the prior art which detect a loss of the input source voltage into the drive circuit, and which act to modify the operation of the circuit and thus of the AC induction motor. One such scheme is found in U.S. Pat. No. 4,673,858 which issued on Jun. 16, 1987 to Saito. In this system, upon detection of a collapse of the input AC source voltage, the torque-producing component of output current to the AC motor is forced substantially to zero such that generally no power is supplied to the load. Therefore little power is drawn from the capacitors, except for maintaining the motor magnetizing current and for supplying the losses inherent in the motor and in the drive circuits. The AC induction motor is allowed to continue rotating in a "free run" condition, so that its speed tends to decrease by natural deceleration.
During the collapse of input source voltage, it is desired to maintain the charge on the capacitors as near to the normal level as possible, both to sustain the control circuit power and so that the drive circuit can resume normal operation quickly upon recovery of the input source voltage. Therefore, it is desired to minimize the loss of charge within the capacitors while the AC induction motor continues to rotate due to inertia after such loss of input source voltage. It is also desirable to provide a drive regulator circuit, which minimizes energy flow from the capacitors when the input source voltage is below a minimum acceptable range.
As described, the prior control method proposed by Saito stops the flow of power out of the motor by clamping the torque command substantially to zero. However, this method still allows the inherent losses in the drive and in the motor to gradually deplete the energy stored in the drive capacitors during the disturbance.
It is therefore an object of this invention to avoid depleting the energy stored in the drive capacitors by using kinetic energy extracted from the load to supply substantially all of the inherent losses, and hence to prolong the ability of the drive to tolerate a collapse of supply voltage.
It is also an object of this invention to effect such control without requiring feedback of the DC voltage on the drive capacitors, since in some topologies DC voltage feedback is not easily available.